Digital circuitry is commonplace in today's electronic systems, such as computers, printers, servers, and telecommunications hardware. The complexity of such systems often involves lengthy internal signal lines spanning up to twenty inches or more through printed circuit board tracks or traces and back-planes. One problem with these signal lines is that they generate noise for the associated digital circuitry; they also often act as antennae to couple unwanted noise signals to the digital circuitry. In one example, certain electronic systems incorporate a voltage signal coupled to a field programmable gate array (FPGA) along a voltage signal line, to indicate power quality; however noise coupled to the signal line may generate a false reading in the FPGA, resulting in unwanted shut-off or other malfunction.
More generally, noise coupled to digital signal lines can create false triggers and metastability problems when a noise spike is latched into a digital circuit. This may happen, for example, when the noise spike coincides with a clock edge in the digital circuit.
Noise in digital circuits may also cause particular problems for signals with slow rise or fall times: when a slowly changing signal is transitioning on a signal line to a digital circuit, noise on that line may result in false detections, by the digital circuit, of multiple edges.
The prior art incorporates several techniques to filter noises on signal lines to digital circuits. A particularly popular approach is to incorporate an analog filter near to the input of the digital circuitry. For example a low pass filter may be implemented at the input to remove high frequency spikes on the signal line. Incorporating low pass filter however also slows down the desired signal edges, which may induce other problems. Analog filters have other problems in that they may utilize resistors and capacitors that are relatively expensive to incorporate on each signal line. An approach utilizing a series of analog filters also typically requires lists and tracking to facilitate configuration management and best practices design engineering, adding additional costs. Stray inductances and capacitance may also induce unwanted resonances within the underlying digital circuit, creating further difficulties.
The prior art has also attempted to filter noises on signal lines to digital circuits by incorporating hysterisis, as with a Schmidt trigger; however, Schmidt trigger devices are susceptible to large voltage spikes, creating unpredictable operation.
The prior art has also utilized the microprocessor to sample the signal line to digital circuitry. The microprocessor may for example pass along a signal line value to the digital circuitry when sampling of the signal line provides a statistically stable line value. Sophisticated versions of this technique may include sampling the signal line at varying frequencies in an attempt to de-couple the sampling from any signal line harmonics. However, systems that incorporate such microprocessors incorporate an expensive and complicated overhead, particularly when the processor is dedicated for this purpose. Furthermore, similar to the low-pass filter problems described above, the delay caused by sampling of the signal line acts as a lag to signal acquisition to the underlying digital circuit. In addition, the electrical designer of the system must meaningfully manage the many processor cycles used in sampling the signal line.
One other popular approach in the prior art to filter noise on digital signal lines, input to an accompanying digital circuit, is the use of cascaded D flip-flops. In this approach, every input clock cycle is clocked into the first D flip-flop, and then progresses down the chain of D flip-flops. After a sufficient number of clock cycles—typically corresponding to the length of the D flip-flop chain—the input is sampled and fed to the digital circuit if all the outputs of the D flip-flops are the same. A significant problem with this approach is that a large number of flip-flops is often required, adding design complexity and cost, and decreasing board real estate available for core system components.
It is, accordingly, one object of the invention is to provide methods and apparatus for filtering signals on a signal line so that noise pulses are not latched into the accompanying digital circuit, and without the afore-mentioned problems. Another object of the invention is to provide a digital filter without the use of analog components or microprocessors. Yet another object of the invention is to provide a method for ensuring a single transition to digital circuitry by filtering unwanted noise components on a signal line to the circuitry. Other objects of the invention are apparent within the description that follows.